Fix sequence number used by asterisk for outgoing RTP packets

There was no audio for 3-way conference when sRTP is used.
For 2-way calls frame->seqno is taken from DSP and is used by asterisk for the sequence number
in RTP headers. However for 3-way conference the sequence number is generated by asterisk and
it has to be greater than the previous value, otherwise libsrtp refuses to forward 'too old'
RTP packets.